In recent years, the computer memory industry is pursuing alternative memory types to replace or supplement memory devices such as dynamic random access memory (DRAM), Flash memory, etc. One of these alternative memory types is conductive bridging random access memory (“CBRAM”), also known as programmable metallization cell (“PMC”) memory. A variety of CBRAM cells and methods of forming them are known in the art.
As an example, a CBRAM cell conventionally includes opposing electrodes on opposite sides of an active material, such as a chalcogenide glass or an oxide glass. The active material is commonly referred to as a solid electrolyte material. A first electrode (e.g., the inert electrode) includes a relatively inert metal (e.g., tungsten) and a second electrode (e.g., the active electrode) includes an electrochemically active metal (e.g., silver or copper). In such a conventional CBRAM cell, the second electrode functions as an ion source material. The resistance of the active material may be changed by applying a voltage across the opposing electrodes. Upon application of a voltage across the electrodes with a positive bias on the second electrode, silver or copper cations drift from the second electrode into the active material and are electrochemically reduced by electrons from the negatively-charged first electrode. The reduced silver or copper atoms are electro-deposited on the first electrode, and the electro-deposition process continues until the silver or copper atoms form a path of less resistance (also referred to as a “conductive bridge,” “dendrite,” or “filament”) across the active material. The conductive bridge may remain in place for an indefinite period of time, without needing to be electrically refreshed or rewritten. Therefore, CBRAM may be referred to as a non-volatile memory.
The formation of the conductive bridge may be reversed, however, by applying a voltage with a reversed polarity (compared to the voltage used to form the conductive bridge) to the electrodes. When a voltage with a reversed polarity is applied to the electrodes, the metallic atoms or atomic clusters that form the conductive bridge are oxidized and migrate away from the conductive bridge into the active material and eventually to the second electrode, resulting in the removal of the low resistance path. Data in the CBRAM cell may be “read” by measuring the resistance between the electrodes. A relatively high resistance value (due to the lack of a conductive bridge between the opposing electrodes) may result in a certain memory state, such as a “0.” A relatively low resistance value (due to the presence of a conductive bridge between the opposing electrodes) may result in a different memory state, such as a “1.”
A CBRAM cell is formed by positioning an active material between a first electrode (e.g., an inert electrode, a bottom electrode) and a second electrode (e.g., an active electrode, a top electrode). The first electrode may be formed of any sufficiently inert conductive material, such as tungsten, titanium nitride, tantalum nitride, etc. The active material conventionally includes a chalcogenide (i.e., including sulfur, selenium, or tellurium) glass or an oxide glass. The second electrode may be formed of an active metal, such as silver or copper, or may be formed of a combination of a conductive ion source material and an inert metal cap of, for example, tungsten, titanium nitride, tantalum nitride, etc. The ion source material, if present, conventionally includes an active metal species (e.g., silver or copper) and is formed between the active material and the metal cap. The ion source material may provide ions for forming a conductive bridge across the active material when proper voltage is applied to the CBRAM cell.
In conventional configurations, an interface between the metal cap and the ion source material, when present, may be characterized by physisorption and little intermixing between the elements at the interface. Internal stresses in the materials of the CBRAM cell, combined with the characteristics of the interface, may cause at least one of adhesive failure and deformation of materials in the CBRAM cell. Poor adhesion and deformation cause problems with or prohibit device fabrication and memory cell performance. Accordingly, a CBRAM cell that reduces or eliminates adhesive failure and deformation is disclosed.